Radiation-curable chemistry has been instrumental in achieving the industrial regulation goals of zero or low volatile organic content (VOC) coatings. UV-curable coatings have successfully replaced solvent-borne technologies for many applications like imaging, graphics, inks, electronic materials, adhesives and sealants, and coatings for surface modification. Since the coatings are cured by UV radiation, the crosslinking reactions take place at room temperature. This has allowed for wide use of UV-curable coatings on heat sensitive substrates like paper, wood and plastic. Most UV-curable formulations have low-viscosity while maintaining 100% solids, which is very difficult to achieve with the traditional solvent-borne or water-borne coatings.
The main components of a typical free-radical UV-curable system include a photoinitiator, an oligomer resin and a reactive diluent. Property-enhancing additives can also be included in the formulation depending on the coating requirements.
The most commonly used UV-curable technology is based on acrylate functional oligomer resins and diluents. A variety of acrylate-functional oligomer resins can be used including urethane-acrylate resins, epoxy-acrylate resins, polyether-acrylate resins, and polyester-acrylate resins. Acrylate functional diluents are low molecular weight compounds such as hexanedioldiacylate, trimethylolpropane triacylate, for example. In a donor-acceptor UV-curing system, an unsaturated polyester resin is the oligomer resin and the diluents can be vinyl ether functional compounds.
Two main methods for radiation curing are electron beam (EB) and ultraviolet (UV) radiation. For EB-curing, the initial step is ionization and excitation of the coating resins by high energy electrons [H. R. Ragin. “Radiation-curable coatings with emphasis on the graphic arts.” Radiation Curing 1992, 273-299]. In case of UV-curing, the first step is the excitation of a photoinitiator or photosensitizer by absorption of photons of LTV-visible electromagnetic radiation. The EB system is different from UV-curable systems due to the absence of any photoinitiating or sensitizing moieties. In this work. LTV electromagnetic radiation has been used as the primary source for curing coatings.
Photoinitiators play a very important role in any successful UV-coating formulation. Since oligomers or reactive diluents do not produce initiating species with a sufficient quantum yield on light exposure, photoinitiators are required for initiating the polymerization [J.-P. Fouassier. Photoinitiation, Photopolymerization and Photocuring Fundamentals and Applications; Hanser Gardner: Munich Vienna New York, 1995]. Photoinitiators absorb the UV-radiation and get excited to an excited state, followed by photolysis into free radicals. The free radicals then attack the unsaturated functional groups present in the formulation (e.g., acrylate groups) to start the chain propagation step. For free radical photopolymerization the most commonly used photoinitiators are Norrish Type I (fragmentation process) or Norrish Type II (hydrogen abstraction) [Y. Chen; J. Loccufier; L. Vanmaele; E. Barriau; H. Frey. “Novel multifunctional polymeric photoinitiators and photo-coinitiators derived from hyperbranched polyglycerol.” Macromolecular Chemistry and Physics 2007, 208, 1694-1706; Y. Chen; J. Loccufier; L Vanmaele; H. Frey. “Novel multifunctional hyperbranched polymeric photoinitiators with built-in amine coinitiators for UV curing.” Journal of Materials Chemistry 2007b, 17, 3389-3392]. Norrish Type I reactions occur through fragmentation or α-cleavage process in which the radicals formed can directly initiate polymerization (Scheme 1).
Scheme 1. Mechanism for Homolytic Fragmentation Type Free-Radical Photoinitiator.
In the Norrish Type II reaction, the free radicals are formed by hydrogen abstraction or electron extraction from a second compound that becomes the actual initiation free radical. This type of photoinitiator requires the presence of a synergist to enable electron transfer and hydrogen abstraction (tertiary amines, amides and ureas). The mechanism for Norrish Type II photoinitiator is outlined in Scheme 2.
Scheme 2. Mechanism for Hydrogen Abstraction Free-Radical Photoinitiator.
Commonly used chemical systems in free-radical radiation curing technology are acrylates, thiol-ene, donor-acceptor systems based on unsaturated polyesters. The present invention relates to donor-acceptor chemistry based on siliconized unsaturated polyester (SUPE)-vinyl ether system.
The use of unsaturated polyesters (UPE) for radiation curing are known [Y. B. Kim; H. K. Kim; H. C. Choi; J. W. Hong. “Photocuring of a thiol-ene system based on an unsaturated polyester.” Journal of Applied Polymer Science 2005, 95, 342-350]. Styrene is often used as a reactive diluent along with a photoinitiator for UPE based radiation-curable systems. For unsaturated polyesters, diacids and diacid anhydrides such as fumaric acid, maleic acid or maleic anhydride are commonly used to incorporate unsaturated moieties in the polyester backbone. When unsaturated polyester having electron deficient moieties is used in combination with vinyl ether having an excess electron charge as a crosslinking mechanism for coating, a donor-acceptor complex is formed. Scheme 3 depicts the most commonly selected donor-acceptor pairs for UV-curable coatings.

Donor-acceptor polymerization is a type of chain growth polymerization of the vinyl groups initiated by free radicals. The maleate-vinyl ether system is the most common donor-acceptor system, wherein vinyl ether is the donor and maleate ester is the acceptor N. Ravindran; A. Vora; D. C. Webster. “Effect of polymer composition on performance properties of maleate-vinyl ether donor-acceptor UV-curable systems.”JCT Research 2006, 3, 213-219]. One of the main advantages of using donor-acceptor UV-curable chemistry is the absence of acrylates, which are known skin sensitizers [R. J. Dearman; C. J. Betts; C. Farr; J. McLaughlin; N. Berdasco; K. Wiench; K. Kimber. “Comparative analysis of skin sensitization potency of acrylates (methyl acrylate, ethyl acrylate, butyl acrylate and ethylhexyl acrylate) using the local lymph node assay.” Contact Dermatitis 2007, 57, 242-247].
Several studies in the area of maleate-vinyl ether chemistry have been previously reported [S. C. Lapin; G. K. Noren; J. M. Julian. “Photoinduced copolymerization of unsaturated ethers with unsaturated esters.” Polymeric Materials Science and Engineering 1995, 72, 589-590; S. C. Lapin; G. K. Noren; J. J. Schouten. “Non-acrylate reactive diluents and oligomers for UV/EB curing.” RadTech Asia '93 UV/EB Conf. Expo., Conf. Proc. 1993, 149-156; R. H. Marchessault; H. Morikawa; J. F. Revol; T. L. Bluhm. “Physical properties of a naturally occurring polyester: poly(b-hydroxyvalerate)'poly(b-hydroxybutyrate).”Macromolecules 1984, 17, 1882-1884]. Lapin et al. studied the properties of oligomers with different backbones and reactive diluents with varying functionalities to formulate UV-curable coatings [S. C. Lapin; G. K. Noren; J. J. Schouten. “Non-acrylate reactive diluents and oligomers for UV/EB curing.” RadTech Asia '93 UV/EB Conf. Expo., Conf. Proc. 1993, 149-156]. Significant work has also been reported on the mechanistic and stereochemical aspects of donor-acceptor chemistry [T. Kokubo; S. Iwatsuki; Y. Tamashita. “Studies on the Charge-Transfer Complex and Polymerization. XVII. The Reactivity of the Charge-Transfer Complex in Alternating Radical Copolymerization of Vinyl Ethers and Maleic Anhydride.” Macromolecules 1968, 1, 482-488; G. S. Prementine; S. A. Jones; D. A. Tirrell. “Model copolymerization reactions. Evidence against concerted complex addition in reactions of simple alkyl radicals with N-phenylmaleimide and donor olefins.” Macromolecules 1989, 22, 770-775].
The incorporation of polydimethylsiloxane (PDMS) segments into polymers and coatings resins is well-studied [D. P. Dworak; H. Lin; B. D. Freeman; M. D. Soucek. “Gas permeability analysis of photo-cured cyclohexyl-substituted polysiloxane films.” Journal of Applied Polymer Science 2006, 102, 2343-2351; H. Ni; W. J. Simonsick; A. D. Skaja; J. P. Williams; M. D. Soucek. “Polyurea/polysiloxane ceramer coatings.” Progress in Organic Coatings 2000, 38, 97-110].
Functional PDMS block copolymers have been previously synthesized to formulate crosslinked hydrophobic coatings [A. Ekin; D. C. Webster. “Synthesis and Characterization of Novel Hydroxyalkyl Carbamate and Dihydroxyalkyl Carbamate Terminated Poly(dimethylsiloxane) Oligomers and Their Block Copolymers with Poly(ε-caprolactone).” Macromolecules 2006, 39, 8659-8668; A. Ekin; D. C. Webster; J. W. Daniels; S. J. Stafslien; F. Casse; J. A. Callow; M. E. Callow. “Synthesis, formulation, and characterization of siloxane-polyurethane coatings for underwater marine applications using combinatorial high-throughput experimentation.” Journal of Coatings Technology and Research 2007, 4, 435-451; P. Majumdar; A. Ekin; D. C. Webster. “Thermoset siloxane-urethane fouling release coatings,” ACS Symposium Series 2007, 957, 61-75]. Due to the low surface energy of PDMS, small amounts of PDMS incorporated into the coating system can provide a hydrophobic surface to the coating.
UV-curable siloxane polymers have also been synthesized for various applications [F. Ferrero; M. Periolatto; M. Sangermano; M. B. Songia. “Water-repellent finishing of cotton fabrics by ultraviolet curing.” Journal of Applied Polymer Science 2008, 107, 810-818; J. He; L. Zhou; M. D. Soucek; K. M. Wollyung; C. Wesdemiotis. “UV-curable hybrid coatings based on vinyl-functionalized siloxane oligomer and acrylated polyester.” Journal of Applied Polymer Science 2007, 105, 2376-2386].
Crivello et al. have synthesized a series of cycloaliphatic and oxetane functional PDMS macromonomers for cationic UV-curing. J. V. Crivello; NI. Jang. “Synthesis of novel silicon-containing monomers for photoinitiated cationic polymerization.” ACS Symposium Series 2007, 964, 27-36; M. Jang; J. V. Crivello. “Synthesis and cationic photopolymerization of epoxy-functional siloxane monomers and oligomers.” Journal of Polymer Science, Part A: Polymer Chemistry 2003, 41, 3056-3073.
Silanol terminated PDMS has been attached to hydroxyl functional polyesters previously. See J. Yang; S. Zhou; B. You; L. Wu. “Preparation and surface properties of silicone-modified polyester-based polyurethane coats.” JCT Research 2006, 3, 333-339. These polyesters have been used for synthesis of segmented polyurethanes with the polyester being the soft segment. However, it is well known that the hydrolytic stability of silyl ether bonds formed from the reaction of a silanol-terminated PDMS and alcohols is poor.
Even with the development of resins having siloxane functionality, there is a need for solvent-free energy curable coatings having low surface energy which do not contain acrylates. These types of low surface energy coatings can be used for applications such as release paper, anti-graffiti coatings, and non-fouling coatings. This invention answers that need.